Alkaline dry batteries generally have an inside-out structure. That is, a cylindrical positive electrode with a hollow is disposed in a battery case serving as the positive electrode terminal so as to closely adhere to the inner face of the battery case, and a negative electrode is disposed in the hollow with a separator interposed therebetween. Manganese dioxide powder is used as the positive electrode active material. Natural manganese dioxide (NMD), chemical manganese dioxide (CMD), or industrially produced manganese dioxide such as electrolytic manganese dioxide (EMD) is used as manganese dioxide. Among them, electrolytic manganese dioxide is used advantageously. Electrolytic manganese dioxide usually contains moisture, ash, and other unavoidable components, with the manganese dioxide (MnO2) purity being a little more than 90%.
Recently, alkaline dry batteries have been required to provide higher performance and, at the same time, there is an increasing demand for low-priced products having high cost performance. To meet such requirements, attempts have been made, for example, to decrease the amount of the positive electrode active material and increase the porosity of the positive electrode so as to increase the amount of electrolyte contained in the positive electrode for enhancing reaction efficiency.
For example, Japanese Laid-Open Patent Publication No. Hei 10-144304 proposes an alkaline dry battery using a positive electrode with a core formation density of 2.9 to 3.1 g/cc. This positive electrode is produced by adding a water-soluble binder such as polyacrylic acid to a mixture of manganese dioxide (positive electrode active material) and graphite (conductive material), and the amount of the water-soluble binder is 0.6 to 1.5% by weight of the manganese dioxide.
Also, Japanese Laid-Open Patent Publication No. Hei 09-180708 proposes an alkaline dry battery using a positive electrode pellet containing manganese dioxide and a conductive material. The positive electrode pellet has pores with pore sizes of 3 nm to 400 μm, and the amount of the pores is 0.14 cc/g or more and 0.24 cc/g or less per unit weight.
In Japanese Laid-Open Patent Publication No. Hei 10-144304, a large amount of a binder is used to produce the positive electrode, in order to prevent a decline in the positive electrode strength due to decreased active material and increased pores in the positive electrode. However, since the large binder amount makes the positive electrode mixture adhesive, the positive electrode mixture tends to adhere to the die or tooling when it is compression molded into a positive electrode pellet or positive electrode. It is thus difficult to produce a predetermined positive electrode pellet or positive electrode. Also, the mold release pressure becomes high and the life of the molding machine tends to become short.
Also, in Japanese Laid-Open Patent Publication No. Hei 09-180708, the apparent density of the positive electrode pellet is approximately 2.6 g/cc, so the strength of the positive electrode pellet is very low. Thus, in the production process, i.e., during the transportation of the positive electrode pellet or in the remolding process inside the battery case, the positive electrode pellet tends to collapse, and the positive electrode pellet is difficult to handle.
In order to solve the above-discussed problems with conventional art, an object of the invention is to provide an alkaline dry battery having a low manganese dioxide density in the positive electrode but having excellent discharge performance, and a method for producing such an alkaline dry battery.